Current trends in microwave electronics have created the need for high frequency and wide bandwidth elements fabricated with integrated circuit technology. Such elements are typically based on transmission line structures. Transmission-line structures are characterized by their characteristic impedance or effective impedance, which determines matching to other structures, and their wave propagation. The effective impedance value is relativity constant over a wide frequency range. One approach to transmission line structures is the use of differential elements.
Differential elements employ balanced transmission line structures as can be seen in FIG. 1A and FIG. 1B. The conventional balanced transmission line structures 100 (FIG. 1A) consists of a top metal layer for the positive line 101 and negative signal line 102, and a bottom metal layer for the ground plane 110. Additionally, a side-shielded balanced transmission line structure 150 (FIG. 1B), consists of a top metal layers for the positive signal line 151, negative signal line 152, a bottom metal layer for the ground plane 160, and the positive signal line side grounded side-shield 153 and negative signal line side grounded side-shield 154.
Differential transmission line structures have an odd-mode effective impedance and an even-mode effective impedance. Typically, the odd-mode effective impedance and the even-mode effective impedance values are similar. However, in balanced elements, it is desired to have an improved properties (e.g. propagation or matching) of the differential signal (odd-mode), and degraded properties for the common signal (even-mode), in order to have good common mode rejection ratio (CMRR).
A balanced element based on differential transmission lines that would have high even-mode effective impedance (thus degrading the even-mode propagation or matching), and would maintain the desired odd-mode effective impedance, would show good differential properties (CMRR) over a wide frequency bandwidth.
There are several passive structures that are based on differential transmission lines, such as line couplers, lange couplers, hybrid couplers, parallel coupled line filters.
Another example for such a structure is the Marchand balun, which is generally used to convert unbalanced transmission line inputs into one or more balanced transmission line outputs or vice versa. The Marchand balun consist of coupled half-wavelength and two quarter-wavelength transmission line structures. Its operation is determined according to usable wavelengths, thus its frequency band is fundamentally narrow.
Other design schemes have been attempted to create wide bandwidth RF passive devices. These approaches include differential transmission lines with slow waves, which creates asymmetry between the even-mode and odd-mode effective impedance but they are very hard to simulate correctly, and therefore implement in integrated circuits. Another approach is the artificial transmission lines that can achieve greater bandwidths but require larger area, exhibits a ripple in its frequency response, and needs complex design.
Thus, a method for increasing bandwidth of differential passive elements is needed.